(1) Field of the Invention
The present invention relates to the general technical field of providing assistance in piloting rotary wing aircraft, and in particular to the fields of obstacle avoidance warnings. More particularly, the present invention relates to a method of issuing a warning to enable a rotary wing aircraft to avoid terrain, and it also relates to a device for performing the method and to an aircraft including such a device.
(2) Description of Related Art
Such piloting assistance systems for issuing a terrain avoidance warning are known as terrain avoidance warning systems (TAWS).
While dangerous obstacles and relief situated ahead of the trajectory of an aircraft are being approached, a TAWS serves to indicate their presence. TAWS systems include in particular a function for avoiding the relief being overflown that is known as forward-looking terrain avoidance (FLTA). Because of this FLTA function, such a TAWS systems makes it possible for example to prevent an aircraft crashing into the ground even though the pilot at the controls has not lost control, e.g. as a result of a navigation error or as a result of being wrongly informed about the height of relief being overflown or of obstacles, if any. Such an accident is sometimes referred to as controlled flight into terrain (CFIT).
Such a system thus makes it possible to produce warnings automatically as a function of databases concerning the relief and obstacles if any being overflown and also possibly as a function of an avoidance trajectory when the trajectory of the aircraft interferes with the relief or indeed with an obstacle.
Another system is known as a ground proximity warning system (GPWS) and serves to warn the pilot of an aircraft that the ground is close.
These systems have been designed mainly for airplanes and they are not well adapted or suitable for rotary wing aircraft. In comparison with a fixed wing aircraft, such as an airplane, a rotary wing aircraft is capable of performing a wide variety of different types of flight. It is only in cruising flight that they are genuinely comparable with the flight of airplanes. A rotary wing aircraft may in particular perform hovering flight, flights in a direction that is purely vertical or purely lateral, and it may also fly for long periods of time very close to the ground and to obstacles.
During such flights, the parameters taken into consideration and the warnings supplied by a terrain avoidance system designed for an airplane are not appropriate and are possibly even undesirable for rotary wing aircraft, since they might constitute a nuisance for the pilot.
In addition, since the controls for piloting a rotary wing aircraft are fundamentally different from those of an airplane, executing an avoidance maneuver requires specific piloting actions to be taken that are different from those in an airplane.
Given this observation, systems for assisting piloting by providing terrain avoidance warnings have been adapted for rotary wing aircraft and are known as helicopter terrain avoidance warning systems (HTAWS).
Document FR 2 773 609 describes a method of avoiding collision between an aircraft and relief being overflown, which method is capable of determining a bundle of possible trajectories for the aircraft together with two avoidance trajectories when performing known maneuvers. An outline can then be obtained and is represented by the intersection between the relief and firstly the possible trajectories of the aircraft or secondly one or two avoidance trajectories. In addition, the outline and the trajectories may be displayed on display means, with color marking representing the risks of the aircraft colliding with the relief.
Also known is Document U.S. Pat. No. 6,380,870 which makes it possible to determine a distance over which the environment and the relief being overflown by an aircraft is analyzed for use in a relief proximity warning system. The analysis distance is variable. The reaction time is the product of a reaction time for the crew multiplied by the speed of advance of the aircraft below a predetermined cruising speed for the aircraft, and is constant above that predetermined cruising speed in order to limit unwanted warnings, in particular at low forward speeds.
According to Document FR 2 813 963, a risk of collision between an aircraft and the relief being overflown is established as a function of static and dynamic parameters of the aircraft and on the basis of at least one database of the relief being overflown. Display means serve to display a representation of that relief, possibly together with warnings and information for avoiding such a collision. Furthermore, the stages of takeoff and landing are taken into account in order to inhibit potential inappropriate warnings.
Furthermore, Document U.S. Pat. No. 6,906,641 describes a relief proximity warning system that defines two flight envelopes for an aircraft in order to improve the generation of warnings. A first envelope is a function of the trajectory of the aircraft, of distance from the relief, of altitude, and of the relief, whereas a second envelope is a function of solely of the trajectory and the distance from the relief.
Furthermore, Document U.S. Pat. No. 6,583,733 describes a relief proximity warning system for a helicopter with two modes of operation. Each mode is selected by the pilot and serves to adapt the system to flying conditions, such as altitude. The detection of relief may thus be improved and the nuisance rate due to inappropriate warnings can be reduced.
According to Document U.S. Pat. No. 6,833,797, a method serves to display the proximity of relief on display means by using color codes as a function of the distance of the aircraft and its forward speed. The flying stage of the aircraft is also taken into account in order to avoid untimely triggering of inappropriate warnings concerning the proximity of relief, in particular while landing.
Furthermore, Document U.S. Pat. No. 6,683,556 describes a method enabling the position and the direction of an aircraft to be determined relative to the relief or an obstacle on the ground. That method serves in particular to determine the height of the aircraft relative to the obstacle or the relief and to use color codes for displaying this height on display means.
Document FR 2 870 604 describes a method of making the flight of an aircraft safe at low altitude. That method monitors the compatibility of the trajectory of the aircraft with the relief being overflown depending on the flying conditions of the aircraft and in compliance with its maneuverability, and in particular its lateral and vertical turning load factors. A trajectory for vertically avoiding the relief can be determined, and for example includes a resource at constant load factor and radius.
Also known is Document EP 1 907 911, which describes a method of detecting a risk of an aircraft colliding with the relief being overflown, during which a trajectory for avoiding the relief is determined. The avoidance trajectory is determined by taking account of energy transfer of the aircraft during the avoidance maneuver and also taking account of variation in the speed of the aircraft.
Document WO 2007/054448 describes a system for providing a warning of an aircraft exceeding a limit point. The limit point corresponds to the last point of the trajectory of the aircraft from which it is possible to succeed in performing a predefined standard maneuver for avoiding the relief, with the avoidance maneuver being a function of the mass, the inertia, and the configuration of the aircraft.
According to Document U.S. Pat. No. 7,363,121, a relief proximity warning system for an aircraft loads a memory with spatial data relating to the relief being overflown by the aircraft in a predetermined priority order for the purpose of providing warnings as quickly as possible to the pilot. The predetermined priority order is a function of the position and of the trajectory of the aircraft.
Document FR 2 932 919 describes a terrain avoidance system for a rotary wing aircraft in which avoidance of the relief is determined while taking account in particular of the instantaneous maneuverability of the aircraft, in particular its instantaneous vertical acceleration and mass. An avoidance trajectory is then determined and is constituted by a substantially rectangular proximal segment representative of a transfer time, together with at least one distal segment that is of conical profile.
Furthermore, Document US 2011/0125346 describes a method defining protection envelopes for an aircraft that are associated with respective different predetermined flight regimes of the aircraft on the basis of a plurality of parameters of the aircraft. Thereafter, a protection envelope is selected as a function of the current flight regime of the aircraft.
Finally, also known as the Document “Design and implementation of TAWS for rotary wing aircraft” by T. Anderson as presented on Mar. 5, 2011 to the “IEEE Aerospace Conference”, and Document US 2010/106419, which Documents form part of the technological background.
The function of issuing a warning to enable an aircraft to avoid terrain is thus generally based on making a comparison in real time between a predicted flight trajectory for the aircraft or an envelope of predicted flight trajectories, and models that represent the relief and obstacles, if any. The predicted trajectory may be associated with one or more predetermined avoidance maneuvers for the purpose of determining a limit point that corresponds to the last point on the predicted trajectory at which such a predetermined avoidance maneuver will enable the aircraft to avoid collision with the relief or the obstacle with a specific safety margin. The limit point is generally referred to as a vertical reference point (VRP).
This function enables a flight trajectory for the aircraft to be followed that is as close as possible to the relief and to give warnings that are appropriate so as to enable the aircraft to perform an appropriate avoidance maneuver in good time in order to avoid a collision.
This function of issuing a warning to enable an aircraft to avoid terrain may operate throughout all the stages of flight of the aircraft, while taking account of the current parameters of the aircraft. Knowledge of the environment being overflown by the aircraft is obtained from databases concerning the relief and the obstacles, if any.
Nevertheless, it can be understood that the variety of possible trajectories that can be performed by a rotary wing aircraft close to the relief and the obstacles makes it difficult to implement such a function of issuing a warning to enable an aircraft of this type to avoid the terrain. The warning needs to be given appropriately in order to leave the pilot enough time to perform the avoidance maneuver. Nevertheless, a definition of the avoidance trajectory that is too approximate may give rise to warnings being issued that are untimely and inappropriate, thereby encouraging the pilot to deactivate the system instead of having confidence in it and making using of it.
Such a function of issuing a warning to enable a rotary wing aircraft to avoid terrain must therefore generate warnings not only at the appropriate moment, but while also remaining reliable and safe in terms of the ability to avoid that relief.
Defining an avoidance maneuver that is realistic, optimal, and suitable for the aircraft is therefore of great importance in ensuring that this function is performed reliably and effectively. Nevertheless, for each model of rotary wing aircraft, maneuverability varies in non-negligible manner as a result of a variety of situations.
In particular, the maneuverability of a rotary wing aircraft can vary as a function of numerous parameters, including:
its intrinsic characteristics, such as its installed engine power, its aerodynamic characteristics, or its configuration, for example;
its flying environment, such as the current density of air; and
its current operating states and parameters such as its instantaneous mass, and its current available engine power, for example.